Wicking Garden Bed

I’ve been looking for some ideas to re-imagine our backyard in the last couple of months, having recently decided to stay in this house for a few years. Inspiration came in a short article in a Green Architecture magazine, talking about these new-fangled wicking garden beds. This discovery, plus a desire to get some growing space under glass, lead to this little project.

Theory

Wicking garden beds are an extension of the idea of the self-watering pot-plants which gained popularity in the 1980s. The idea is to provide a reservoir of water under the plants to buffer the changes in moisture during the day, or to cover you when you forget to water.

Growing under glass is a separate idea, designed to extend the growing season, particularly in cold climates. Here in Canberra, frosts still come quite late in the year and have a habit of killing tomatoes, and making eggplant, capsicum and chilli almost impossible to grow. To bring planting forward and give the plants a chance of a long growing season, growing under glass is a simple way to keep the frosts off. Glass also lets the sun light through, heating the soil more than the open beds, and triggering earlier germinatiopn.

We had some glass replaced in the house recently, so I had a window lying around looking for a project. So, combine this, the article on wicking garden beds and some beautiful September sunshine and you’ve got a weekend gardening project.

Materials
Here they are:

I used timber for the frame, mostly because it’s so easy to work with. I won’t use treated pine for vege beds any more, worrying about the curative agent coming into contact with food, but in this job it works quite well. The lining means the timber doesn’t come in contact with the growing medium, so you don’t really have anything to worry about. Timber also makes the various brackets and staples much easier to add later.

So, beyond the frame material, you will need:
Pool/pond liner/builder’s plastic
Scoria/gravel/broken bricks
Sand
Shade cloth/geotextile
Dirt/poo
Some 65mm PVC pipe
Some sort of adhesive/sealant. I used polyurethane
Some screws and things.
Relevant tools

The glass on top isn’t necessary for the bed, but to fit it you’ll need some hinges.

Method

Build your garden bed frame. You want it to be somewhere between 450 and 600mm deep. Common wisdom is that plants won’t wick water up more than 300mm, so that’s your maximum depth of dirt. 150mm of resorvoir is about what I used, I think much deeper than that would be a lot of water.

I made two boxes, then stacked them on top of each other.

The glass weighs a bit, so I fastened the two boxes together pretty carefully, so the top one can’t skid off. That’s just a bracket I had lying around.

I didn’t do it this way, but you should put the hole in the box for the drainage pipe now. It’s too hard to do after the liner is in.

This galvanised building strapping was quite useful; again just something I had left over from a previous project.

Next, make a nice flat bottom inside the box using the sand.

You could dig out the grass if you’re keen, but I didn’t bother. Sand straight on top, then levelled with a piece of wood.

Once you’ve got a flat bottom, line the box with the plastic. This took a bit of fiddling, but this part is worth getting right.

I secured the top around the edge of the box, using the galvanised strap.

The gravel in there is just to make sure the plastic is sitting properly.

Detail of the galvanised strap holding the plastic in place

Now, drill a lot of holes in your PVC pipe, and cut it so it sticks out the hole in the side by about 100-200mm. The next step is crucial.

Make sure you get a good seal all the way around the pipe, where it goes through the plastic. This should be the lowest point of the reservoir and if it doesn’t hold water, all is lost. I did a terrible and ugly job, but was pretty keen to get a good seal. This photo was actually taken before I finished, as I put on gloves and doubled the amount of sealant after this.

Fill with gravel, to a depth of 150mm or so.

The water gets in between the gaps in the rocks. The rocks support the weight of dirt above.

Add layer of shade cloth. This stops the dirt getting between the rocks. I secured the cloth all the way around with the metal strapping, mostly so when I pull tomatoes out at the end of the season, the roots don’t disturb the cloth.

Fill with dirt and manure. Make it good stuff. You’re growing food in here, so you want high organic content and loads of nitrogen. I used about 50/50 potting mix and cow manure.

Take a break because baby got bored and started crying.

Fit glass

Remember glass is heavy and dangerous, so space the hinges sensibly and make sure the frame can handle the load. I used glass in a frame which was spare from some renovations, but I suspect you could get a similar frame for $50 from a building material recycler or tip-shop.

You can also see here the elbow which governs the maximum water depth, and allows inspection of the water quality. Having that screw-on drainage plug on the outside means I can drain it dry if I want.

I fitted some struts to hold the glass up when I’m working on the veges. Make sure it hinges toward north so you don’t shade your veges later in the season. The bracing should again be well spaced and bare the load evenly. Also, make sure you pin both connections, rather than latching it to sit on one direction. It would be very messy to have the glass flip over in a high wind.

Plant seeds. Mulch heavily.

Water from above for the first few weeks as the roots won’t go very deep yet. I am hoping the roots will ‘smell’ the water and head straight for it.

Conclusion

This looks like it will be a pretty good system, but only time will tell. I’m expecting good results in Canberra, due to the longer season, and the battering the plants take while I’m at work on hot dry days. On the negative side, this uses a lot more materials than a standard bed, and is cut off from the surrounding soil because of the plastic liner, making it difficult for worms to get in and improve your soil. I hope the cow manure will fix this, as it usually contains some worm eggs.

So, we’ll see how it goes, but I’m expecting big things. I’ll take some more pics later in the season.

EB